A distinction is made between spectacle lens semi-finished products and spectacle lens finished products. Semi-finished products is the term used in the production business for partially finished preliminary products, which a company either puts in store for processing later itself or passes on to other companies to complete production. Spectacle lens semi-finished products are lens blanks with only one optically finished surface (cf. Heinz Diepes, Rolf Blendowske “Optik and Technik der Brille” [spectacle optics and technology], Optische Fachveröffentlichung GmbH, Heidelberg, 2002, page 560). Finished products is the term used for products that are finished and ready for sale, which a company either delivers immediately or puts in its own store for sale later. Spectacle lens finished products or finished spectacle lenses are spectacle lenses with two optically finished optical surfaces. These may or may not be rimmed at the ends (cf. ibidem, page 559). A rimmed spectacle lens is accordingly a finished spectacle lens that has been brought to the final size and shape by working the rim (cf. ibidem, page 559).
Also often used to describe unfinished spectacle lenses is the term blanks. Blanks are usually preformed pieces of material for making spectacle lenses in some state or other before completion of the surface finishing (cf. ibidem; page 556). The term semi-finished blanks is a synonym for the term semi-finished products.
Spectacle lens semi-finished products and spectacle lens finished products have optical surfaces intended for a spectacle wearer, in each case one for arrangement on the object side and one for arrangement on the opposite, eye side, and a surface keeping these surfaces apart. The optical surface intended for arrangement on the object side is referred to as the front surface, the optical surface intended for arrangement on the eye side is referred to as the rear surface. The surface lying in between, either directly forming an edge or indirectly adjoining by way of an edge surface at one end the front side and at the other end the rear side, is referred to as the cylinder rim surface.
While in the past spectacle lenses were predominantly made from mineral glasses, in particular crown glasses (Abbe number>55) and flint glasses (Abbe number<50), in the meantime spectacle lenses of a large number of organic materials are available.
At present, spectacle lens semi-finished or finished products with spherical, aspherical or progressive front surfaces are cast in mass production processes by means of front- and rear-surface molding shells and a sealing ring, as described for example in the documents U.S. Pat. No. 4,300,821, U.S. Pat. No. 6,103,148 A or JP 2008 191186 A. This applies to materials with the trade names MR-7, MR-8, MR-10 and CR-39/CR-607 and others. The materials with the trade names MR-7, MR-8, MR-10 are polythiourethanes, which are sold by the company Mitsui Chemicals. The abbreviation “MR” stands here for Mitsui Resin. CR-39/CR-607/CR-630 are low-refractive-index materials, with a refractive index of 1.50, which are sold by the company PPG Industries. The materials CR-607 and CR-630 are used for example for photochromic applications.
Semi-finished products of finished products for spectacle lenses of polycarbonate are produced in metal molds by means of an injection-molding technique. This method of making them is described for example in EP 0955147 A1.
Order-specific prescription spectacle lenses, i.e. in particular individualized single-vision and multifocal lenses and in particular variable-focus or progressive lenses, are brought into their final form by mechanical processes. The outer forms may be formed here as round, oval or arbitrary, describing so-called freeforms.
The semi-finished or finished products described above are often subjected to one or more refinement processes. In particular, functional layers are applied on one or both sides. Such functional layers are layers that provide the spectacle lenses with properties which are predetermined and advantageous for the spectacle wearer and which the spectacle lenses would not have just on the basis of their material properties and shaping. Apart from optical properties, such as for example antireflective or reflective coating, light polarization, coloring, etc., such advantageous properties are also mechanical properties, such as hardening, reducing the adherence of dirt or misting, etc., and/or electrical properties, such as shielding from electromagnetic radiation, conduction of electrical current, etc., and/or other physical or chemical properties.
The application of functional layers often takes place with the aid of wet coating processes. In production technology, coating is understood as meaning a main group of production processes conforming to DIN 8580, which are used for applying a firmly adhering layer of formless substance to the surface of a workpiece. Wet coating processes are such coating processes in which the initial state of the coating material is liquid. In the case of spectacle lens production, dip coating processes and spin coating processes in particular are of special importance.
In dip coating processes, to apply the coating spectacle lenses (semi-finished or finished products) are usually pulled out of the dipping bath linearly in the direction of the normal to the front or rear surfaces. In this case, the running off behavior of the coating produces accumulations of coating at the lowest point. If, in addition, a supporting element, such as for example a retention spring, or an adhesive location also affects the running-off behavior of the coating there, the accumulation of coating on the optical surface becomes up to 3 to 4 mm larger. The accumulation of coating reduces the useful optical surface area and/or may lead to rejection for cosmetic reasons.
In spin coating processes, spectacle lenses (semi-finished or finished products) are wetted with coating. The application of the coating is performed for example by means of a metering pin or by horizontal or inclined dipping in a container filled with coating. For even distribution of the coating layer, the optical lenses are rotated at a sufficient speed and the coating is spun off. In the process, a residual amount of coating collects at the transition between the optical surface and the cylinder rim surface and forms a peripheral fringe of coating. The accumulation of coating reduces the usable optical surface area and/or may lead to rejection for cosmetic reasons.
With supporting gas processes, for example flooding with nitrogen during the curing of the coating, turbulences may occur at the transition, and in turn cause structures to form from the coating, which may for example take the form of serrations.
Depending on how the process is conducted, defects such as for example gas bubbles, may form in the coating layer while spin and dip coating processes are being carried out. With linear pulling out or spinning off, these bubbles may attach themselves at the transition between the optical surface and the cylinder rim surface and lead directly to rejection.
When curing the coating applied for example by means of a spin and/or dip coating process, differences in stress may occur. These may lead to cracks in the coating.